When completions menu is displayed above cursor, reverse order (#23446)

Considered doing this when previously working on completions menu
layout, as it brings the default selection position next to the cursor
position, and is generally more symmetrical. With #23445 there is now a
more compelling reason, as the "translucent, cropped bottom" display
doesn't make sense when displayed above.

Release Notes:

- N/A

crates/gpui/src/geometry.rs

@@ -2500,6 +2500,8 @@ impl Pixels {
     pub const ZERO: Pixels = Pixels(0.0);
     /// The maximum value that can be represented by `Pixels`.
     pub const MAX: Pixels = Pixels(f32::MAX);
+    /// The minimum value that can be represented by `Pixels`.
+    pub const MIN: Pixels = Pixels(f32::MIN);
 
     /// Floors the `Pixels` value to the nearest whole number.
     ///